Optical Fibres in ‘Instituteofmaking’

Glasses with a high fluoride content hold the most promise for improving optical fiber performance because they are transparent to almost the entire range of visible light frequencies. This makes them especially valuable for multimode optical fibers, which can transmit hundreds of discrete light wave signals concurrently. An optical fiber is a single, hair-fine filament drawn from molten silica glass.

To make an optical fiber, layers of silicon dioxide are first deposited on the inside surface of a hollow substrate rod. This is done using Modified Chemical Vapor Deposition, in which a gaseous stream of pure oxygen combined with various chemical vapors is applied to the rod. As the gas contacts the hot surface of the rod, a glassy soot several layers thick forms inside the rod. After the soot is built up to the desired thickness, the substrate rod is moved through other heating steps to drive out any moisture and bubbles trapped in the soot layers. During heating, the substrate rod and internal soot layers solidify to form the boule or preform of highly pure silicon dioxide. After the solid glass preform is prepared, it is transferred to a vertical drawing system. In this system, the preform is first heated. As it does so, a gob of molten glass forms at its end and then falls away, allowing the single optical fiber inside to be drawn out. The fiber then proceeds through the machine, where its diameter is checked, a protective coating is applied, and it is cured by heat. Finally, it is wound on a spool. A typical optical fiber cable usually includes several optical fibers around a central steel cable. Various protective layers are applied, depending on the harshness of the environment where the cable will be situated.

The Future

Future optical fibers will come from ongoing research into materials with improved optical properties. Currently, silica glasses with a high fluoride content hold the most promise for optical fibers, with attenuation losses even lower than today’s highly efficient fibers. Experimental fibers, drawn from glass containing 50 to 60 percent zirconiumfluoride (ZrF 4 ), now show losses in the range of 0.005 to 0.008 decibels per kilometer, whereas earlier fibers often had losses of 0.2 decibels per kilometer.

In addition to utilizing more refined materials, the producers of fiber optic cables are experimenting with process improvement. Presently, the most sophisticated manufacturing processes use high-energy lasers to melt the preforms for the fiber draw. Fibers can be drawn from a preform at the rate of 10 to 20 meters (32.8 to 65.6 feet) per second, and single-mode fibers from 2 to 25 kilometers (1.2 to 15.5 miles) in length can be drawn from one preform. At least one company has reported creating fibers of 160 kilometers (99 miles), and the frequency with which fiber optics companies are currently retooling—as often as every eighteen months—suggests that still greater innovations lie ahead. These advances will be driven in part by the growing use of optical fibers in computer networks, and also by the increasing demand for the technology in burgeoning international markets such as Eastern Europe, South America, and the Far East.

Optical fibres carry telecommunication signals in the form of pulses of light that we rely on for phone and internet. Millions of miles of fibres have been laid across the seabed, enabling fast global telphony and the internet access. They’re composed of a solid glass core, which traps and reflects light along it so that the light follows the curve of the optical fibre. Layers of external cladding are often additionally applied to protect the fibre from damage. Because optical fibres can be used to trap light at one end and then emit it at the other, then another application is in key-hole surgery, which sees the use of optical fibres being inserted into the body to allow surgeons to clearly see what’s going on inside.

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